Play in juvenile mink: litter effects, stability over time, and motivational heterogeneity.

Mink are potentially ideal for investigating the functions of play: deleterious effects of early social isolation suggest a crucial developmental role for play; and huge numbers of highly playful juvenile subjects can be studied on farms. We collected descriptive data on 186 pairs from 93 litters, half provided with play-eliciting environmental enrichment objects in their home cages, to test three hypotheses: (1) play frequency is subject to litter effects; (2) relative playfulness is stable over time; (3) play sub-types share a single, common motivational basis. We found weak litter effects that were driven by stronger litter effects on general activity, and weakly stable individual differences in both total and rough-and-tumble play. Experimentally increasing object play did not inhibit rough-and-tumble play, showing these sub-types are not motivational substitutes. Frequencies of these sub-types were also uncorrelated, and changed differently with time of day and age, further supporting this conclusion.